Addressing climate change is a large-scale global challenge. Currently, the world's economies annually emit approximately 26 gigatons of CO2 (GtCO2) to the atmosphere from the combustion of fossil fuels. In the absence of explicit efforts to address climate change, rising global populations, higher standards of living, and increased demand for energy could result in as much as 9,000 gigatons of cumulative CO2 being emitted to the atmosphere from fossil fuel combustion over the next century.
To stabilize CO2 concentrations in the atmosphere “at a level that would prevent dangerous anthropogenic interference with the climate system” as called for in the United Nations Framework Convention on Climate Change, the cumulative amount of CO2 released to the atmosphere over this century would need to be held to no more than 2,600 to 4,600 GtCO2—a substantial reduction and a formidable challenge.
CO2 Capture and Sequestration (CCS) is one of the most important of the mitigation options. CCS involves the separation CO2 from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere. Technologies exist or are under development to capture CO2 but their cost provides a significant barrier to widespread adoption. Transport to sites for alternative use or sequestration is provided by traditional compression and pipeline technologies. Other uses for captured CO2 are for enhanced oil recovery, coal bed methane recovery and food and beverage processing. Current examples of CO2 capture from process streams include the purification of natural gas and production of hydrogen-containing synthesis gas for the manufacture of ammonia, alcohols and synthetic liquid fuels.
The use of CaO (commonly known as burnt lime, lime or quicklime) as a regenerable sorbent for CO2 capture has been proposed in several processes dating back to the 19th century. The carbonation reaction of CaO to separate CO2 from hot gases (T>200° C.) and form CaCO3 is very fast. The regeneration of the sorbent by calcining the CaCO3 into CaO and pure CO2 is favored at T>900° C. (at a CO2 partial pressure of 0.1 MPa).CaO(s)+CO2(g)CaCO3(s)  EQUATION 1
This carbonation-calcination cycle was successfully tested in a pilot plant (40 tonne d−1) for the development of the acceptor coal gasification process using two interconnected fluidized beds (see G. P. Curran, C. E. Fink and E. Gorin, Adv. Chem. Ser. 69 (1967) 141-165). The use of the above cycle (Equation 1) for a post-combustion system was first proposed by Shimizu et al. (I Chem E., 77-A (1999) 62-68) and involved the regeneration of the sorbent in a fluidized bed, firing part of the fuel with O2/CO2 mixtures. The effective capture of CO2 by CaO has also been demonstrated in a small pilot fluidized bed (J. C. Abanades et al. AIChE. J. 50(7) (2004) 1614-1622).
A disadvantage of all of these processes is that the capacity of natural sorbents (limestones and dolomites) to capture CO2 typically diminishes rapidly, and a large make-up flow of sorbent (of the order of the mass flow of fuel entering the plant) is required to maintain the CO2 capture activity in a capture-regeneration loop (J. C. Abanades, E. S. Rubin and E. J. Anthony Ind. Eng. Chem. Res. 43 (2004) 3462-3466). Although the deactivated sorbent may find application in the cement industry and the sorbent cost is low, a range of methods to enhance the activity of calcium-based CO2 sorbents have been pursued.
Abanades and co-workers have published several papers examining the CaO/CO2 system employing sequential calcination and carbonation steps (see: J. C. Abanades et al. “In-situ capture of CO2 in a fluidized bed combustor” Proc. 17th Int. Fluidized Bed Combustion Conference, Jacksonville, Fla., May 2003; J. C. Abanades and D. Alvarez Energy Fuels 17 (2003) 308-315; and J. C. Abanades Chem. Eng. J. 90 (2002) 303-306). They reported that the capacity of limestone to be recarbonated falls continuously with the number of cycles. After examining data from a number of researchers who used different limestones, different particle sizes (10 μm to 10 mm) and a range of treatment temperatures (750 to 1060° C.), they concluded that there was uniformity in the conversion displayed by equivalent data, which defined a generalized correlation. The correlation relating the conversion capacity to the number of the cycle is given as:XN=fmN(1−fW)+fW  (1)where N is the number of the cycle (for uncalcined limestone N=0), and fm is the fractional loss in conversion from the previous cycle, assumed constant with N. The parameter fW is the theoretical residual capacity after infinite cycles. From their collection of data, Abanades and Alvarez (Energy Fuels 17 (2003) 308-315) assigned values of 0.77 to fm and 0.17 to fW.
WO 2005/046863 describes a process for reactivating lime-based sorbents for multiple CO2 capture cycles during the fluidized bed oxidation of combustion fuels by hydrating the lime after each calcination or by shocking the lime with pure CO2.
Fennell et al. (J. Energy Inst. 80 (2007) 116-119) have shown that the drop in CO2 absorption capacity when limestone is subject to at least 10 calcination-carbonation cycles is partially recovered when the lime is exposed to water vapor at ambient temperatures overnight. They also observed that the limestone underwent substantial attrition which they attributed to the regeneration by the hydration step.
The reactivity and durability of calcium-based sorbents for CO2 absorption during repetitive carbonation-calcination reactions at different pressures in a laboratory-scale horizontal-tube reactor has been investigated (K. Kuramoto et al. Ind. Eng. Chem. Res. 42 (2003) 975-981). It was found that the sorbents were significantly deactivated with respect to CO2 absorption by high-temperature calcination treatment as a result of sintering and crystal growth. As a consequence, their CO2 absorption capacity decreases with cycle number in repetitive calcination-carbonation reactions under both atmospheric and pressurized conditions. For at least seven cycles hydration treatment was effective in reactivating these sorbents. Also, the durability of the sorbents in repetitive CO2 sorption was recovered by hydration treatment both at ambient temperature and pressure, and at elevated pressure at 200° C. However, at elevated pressure, the sorbents melted in repetitive calcination-hydration-carbonation reactions at 923 K and 973 K, most likely because of eutectics in the CaO—Ca(OH)2—CaCO3 ternary system.
Accordingly, it is an object of the present invention to go some way to avoiding the above disadvantages or to at least provide the public with a useful choice.
Other objects of the invention may become apparent from the following description which is given by way of example only.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date.